The present invention pertains to forming a film bulk acoustic resonator (xe2x80x9cFBARxe2x80x9d) structure. More specifically, the present invention relates to the methods of forming a plurality of film bulk resonator structures on a substrate and relates to the structure of the film bulk resonator.
In some instances it is desirable to provide a radio frequency front-end filter. Ceramic filters and saw filters are used as front-end radio frequency filters, ceramic filters and saw filters still dominate, but there are problems with ceramic filters and saw filters. Saw filters start to have excessive insertion loss above 2.4 gigahertz (GHz). Ceramic filters are large in size and can only be fabricated with increasing difficulty as the frequency increases.
FBARs have replaced ceramic filters and saw filters in limited cases. The FBARs have better performance than ceramic filters and saw filters. A basic FBAR device 100 is schematically shown in FIG. 1. The FBAR device 100 is formed on the horizontal plane of a substrate 109. A first layer of metal 120 is placed on the substrate 109, and then a piezoelectric layer (AlN) 130 is placed onto the metal layer 120. A second layer of metal 122 is placed over the piezoelectric layer 130. The first metal layer 120 serves as a first electrode 120 and the second metal layer 122 serves as a second electrode 122. The first electrode 120, the piezoelectric layer 130, and the second electrode 122 form a stack 140. A portion of the substrate 109 behind or beneath the stack 140 is removed using backside bulk silicon etching. The backside bulk silicon etching is done using deep trench reactive ion etching or using a crystallographic orientation-dependent etch, such as KOH, TMAH, and EDP. Backside bulk silicon etching produces an opening 150 in the substrate 109. The resulting structure is a horizontally positioned piezoelectric layer 130 sandwiched between the first electrode 120 and the second electrode 122 positioned above the opening 150 in the substrate. The FBAR is a membrane device suspended over an opening in a horizontal substrate.
FIG. 2 illustrates the schematic of an electrical circuit 200 which includes a film bulk acoustic resonator 100. The electrical circuit 200 includes a source of radio frequency xe2x80x9cRFxe2x80x9d voltage 210. The source of RF voltage 210 is attached to the first electrode 120 via electrical path 220 into the second electrode 122 by the second electrical conductor 222. The entire stack 140 can freely resonate in the Z direction (xe2x80x9cD33xe2x80x9d mode) when the RF voltage at resonant frequency is applied. The resonant frequency is determined by the thickness of the membrane or the thickness of the piezoelectric layer 130 which is designated by the letter d or dimension d in FIG. 2. The resonant frequency is determined by the following formula:
f0xcx9cV/2d, where
f0=the resonant frequency,
V=the acoustic velocity in the Z direction, not the voltage, and
d=the thickness of the piezoelectric layer.
It should be noted that the structure described in FIGS. 1 and 2 can be used either as a resonator or as a filter. However, such a structure has many problems. For example, as the thickness of the layers are reduced, then the resonance frequency of the device will be increased. A filter to be used in a high frequency application requires a thin membrane. Thin membrane devices are very fragile.
The backside bulk silicon etching produces a wafer having large openings therein. Wafers with large openings therein are much weaker than a wafer without openings therein. The wafers with large openings therein are much more difficult to handle without breaking.
The membrane device that results also must be protected on both sides of the wafer. As a result, the packaging costs associated with the FBAR membrane devices are higher than a device that must be protected on one side only.
Still a further disadvantage is that backside bulk etching of silicon is a slow process with significant yield problems. In addition, the equipment and processes needed to conduct a backside bulk etching of silicon differs from the equipment and processes used in standard integrated circuit processing which add to the cost of production and is less compatible with standard integrated circuit production.
Thus, there is general need for an FBAR device and a method for producing one or more FBAR devices that is more compatible with standard processes associated with standard integrated circuit processing techniques. The is also a general need for a FBAR device that is more durable. There is still a further need for an FBAR device that can be formed for high frequency applications which does not use as much area of a wafer as current FBAR devices. There is also a general need for a FBAR device that does not have to be protected on both sides so that packaging costs associated with the device are less. There is also a need for a process which keeps the wafers stronger during production so that the wafers are easier to handle during production.